Process for recovering volatilized rhenium oxides and sulfur oxides from gas streams

ABSTRACT

A gas stream containing a silfur oxide and a volatilized rhenium oxide is scrubbed with an aqueous alkaline solution containing ions capable of forming sulfites and bisulfites, e.g., an ammonium or an alkali metal solution, to remove practically all of the sulfur oxide from the gas stream as a soluble sulfite and to dissolve the rhenium oxide in the resulting sulfite solution, which is maintained alkaline for the purpose. The rhenium-bearing sulfite solution is treated by known means, such as ion exchange or solvent extraction, to recover the rhenium oxide contained therein, and the effluent sulfite solution is then employed as the scrubbing solution in a second gas scrubbing stage in which a different sulfur-oxide-containing gas stream, such as the exit gas from a sulfuric acid plant, having substantially no rhenium content, is scrubbed to extract sulfur oxide, making such effluent sulfite solution acid and converting the sulfites into soluble bisulfites. Sulfur values are preferably recovered from the resulting bisulfite solution.

llnited States Ratent 91 Spedden [451 Mar. 27, 1973 PROCESS FORRECOVERING VOLATILIZED RHENIUM OXIDES AND SULFUR OXIDES FROM GAS STREAMS[75] Inventor: Henry Rush Spedden, Salt Lake [58] Field ofSearch..23/l8, 22, 25 Q, 15 W; 423/49, 423/50, 210, 242

[56] References Cited UNITED STATES PATENTS 3,475,122 10/1969 McRae etal. ..23/2 SQ 2,809,092 10/1957 Zimmerl et al. ...23/l5 W 2,967,757l/l96l Zimmerley et ....23/l5 W Primary Examinerl-lerbert T. CarterAttorneyPhilip A. Mallinckrodt et al.

[57] ABSTRACT A gas stream containing a silfur oxide and a volatilizedrhenium oxide is scrubbed with an aqueous alkaline solution containingions capable of forming sulfites and bisulfites, e.g., an ammonium or analkali metal solution, to remove practically all of the sulfur oxidefrom the gas stream as a soluble sulfite and to dissolve the rheniumoxide in the resulting sulfite solution, which is maintained alkalinefor the purpose. The rhenium-bearing sulfite solution is treated byknown means, such as ion exchange or solvent extraction, to recover therhenium oxide contained therein, and the effluent sulfite solution isthen employed as the scrubbing solution in a second gas scrubbing stagein which a different sulfuroxide-containing gas stream, such as the exitgas from a sulfuric acid plant, having substantially no rhenium content,is scrubbed to extract sulfur oxide,'making such effluent sulfitesolution acid and converting the sulfites into soluble bisulfites.Sulfur values are preferably recovered from the resulting bisulfitesolution.

13 Claims, 1 Drawing Figure VOLATILIZED RHENIUM AND SULFUR OXIDE BEARINGGAS AMMONIUM HYDROXIDE SCRUBBING TOWER (ALKALINE pH) AMMONIUM SULFITESOLUTION CONTAINING DISSOLVED RE OXIDES ExIT GAS RHENIUM RECOVERYRHENIUM VALUES AMMONIUM SULFITE SOLUTION EXHAUST GAS TO TAIL SCRUBBINGTOWER BEARING GAS ELECTROSTATIC PRE (AcIOIc pH) (ACID PLANT EXHAUST)CIPITATOR OR STACK AMMONIUM BISULFITE SOLUTION S0 STRIPPER AMMONIUMBISULFITE PRODUCT so As AN END PRODUCT OR To ACID PLANT T AMMONIUMSULFATE SOLUTION INVENTOIL HENRY RUSH SPEDDEN ATTORNE YS PROCESS FORRECOVERING VOLATILIZED RHENIUMOXIDES AND SULFUR OXIDES FROM GAS STREAMSBACKGROUND OF THE INVENTION 1.Field This invention relates to theextraction of rhenium values from gas steams containing same.

2. State of the Art It has long been standard practice to extractvolatilized rhenium oxides from effluent gases of molybdenite roastersby scrubbing the gases in conventional scrubbers with water that isrecycled to build up the rhenium content. The water also dissolvessulfur oxides,- and in the course of the operation becomes saturatedwith sulfur dioxide and strongly acid in character. Following suchsaturation, continuation of the process results in additional extractionof rhenium values from the gas stream but no additional extraction ofsulfur dioxide. Consequently, if the exhaust gas is vented directly tothe atmosphere it constitutes a pollutant. Otherwise, further expensiveprocedures are required for elimination of the sulfur dioxide.

A known process for extracting SO from smelter stack gases employs twoscrubbers through which the sulfur dioxide-bearing gases flowcountercurrent to a scrubbing solution containing ammonium sulfite,which extracts the sulfur dioxide and is converted into ammoniumbisulfite (acid pH) in the scrubber through which the gases first flowand the solution flows secondly. The other scrubber is operated at abasic pH to remove residual amounts of sulfur dioxide contained in theexit gas from the first scrubber. The process has not been employed toextract rhenium values from effluent gas streams containing both rheniumoxides and sulfur oxides. If the process were to be used for thatpurpose, additional amounts of expensive ammonium hydroxide reagentswould be required to neutralize the acidic pH in the first scrubber inorder to obtain satisfactory recovery of the rhenium values from thesolution.

3. Objectives In the making of the present invention, it was anobjective to extract rhenium oxides from an effluent industrial gascontaining same, as well as to recover, as a useful bisulfite product,the sulfur oxides contained in such gas stream, thereby eliminating asource of atmospheric pollution as well as obtaining significanteconomic returns.

SUMMARY OF THE INVENTION In accordance with the invention, a gas stream,containing a rhenium oxide and a sulfur oxide, is intimately contactedwith an aqueous alkaline solution containing cations capable of formingsulfites and bisulfites. Suitable gas streams containing both rheniumand sulfur oxides include the exhaust gases from copper reverberatoryfurnaces and molybdenite roasters. Conventional gas scrubbing apparatus,such as Venturi-type scrubbers, are well suited to utilization in theprocess. Typical cations capable of forming the required sulfites andbisulfites are ammonium ions and alkali metal ions. In the firstscrubbing step, the scrubbing solution extracts the major part, if notall, of the sulfur oxide or oxides, e.g. sulfur dioxide and trioxidefrom the gas stream to form a soluble sulfite. The volatilized rheniumoxide or oxides are dissolved in the alkaline solution and are therebyextracted from the gases.

The effluent solution from the first scrubbing step containing thedissolved sulfites and rhenium oxides, is treated to remove the rheniumoxides from the solution. Known extraction techniques, such as ionexchange or solvent extraction, can be employed for this purpose.

The stripped solution containing the dissolved sulflte or sulfites isemployed as the scrubbing solution in the other scrubbing stage, inwhich a second gas stream containing a sulfur oxide is scrubbed at anacid pH to remove the sulfur oxide or oxides from the gas stream and toprovide the additional moles of sulfite necessary to convert thesulfites to bisulfites. It may be desirable to also run the exhaust gasfrom the first scrubber into the second scrubber to salvage the alkalinereagent contained in mist carried by such exhaust gas and to recover anyresidual sulfur oxides that may have escaped the first scrubber. Theresulting bisulfite solution can be treated by known means to recover abisulfite product, e.g. ammonium bisulfite, or, if desired, can betreated to strip out sulfur dioxide for direct recovery of SO or for theproduction of sulfuric acid, leaving a sulfate product, e.g. ammoniumsulfate, in the solution. The exhaust gas from the other scrubbing stagecan either be cycled through a wet electrostatic precipitator to reducethe mist content or can be vented directly to the atmosphere.

THE DRAWING The best mode presently contemplated of carrying out theinvention is illustrated in the accompanying drawing, in which thesingle FIGURE is a flowsheet showing the process applied to a gas streamcontaining volatilized rhenium oxides and sulfur oxides.

DETAILED DESCRIPTION OF THE ILLUSTRATED PROCEDURES As illustrated in theaccompanying flowsheet, any gas stream containing both volatilizedrhenium oxides and sulfur oxides, such as sulfur dioxide or sulfurtrioxide, can be employed as the input gas in the first step of theprocess. Most commercial sources of volatilized rhenium oxides,including molybdenite roasting operations and copper reverberatoryfurnaces, generate significant quantities of sulfur oxides in additionto the volatilized rhenium oxides.

In the first step of the process, the gas stream containing thevolatilized rhenium oxides and sulfur oxides is intimately contacted atan alkaline pH with an aqueous solution containing cations capable offorming a soluble sulfite, in this instance ammonium ions, in aconventional scrubber, such 'as a Venturi scrubber. Ammonia ispreferably introduced into the aqueous solution prior to the solutioncontacting the gas stream, so that ammonium hydroxide is generated inthe solution to create an alkaline medium. Alternatively, ammoniumhydroxide is conveniently generated in situ within the scrubber byinjecting ammonia gas which contacts the aqueous solution formingammonium hydroxide. Sufficient ammonium hydroxide is present in thesolution to maintain the pH on the alkaline side, preferably betweenabout pH 8 and pH 10. As the ammonium ions react with the sulfur oxidesin the gas stream, the formation of ammonium sulfute causes a reductionin the ammonium hydroxide concentration, requiring the introduction ofadditional amounts of ammonia into the reaction. Although ammonium ionsare employed in the detailed embodiment of the process, other cationscapable of generating hydroxides to control pH and to form sulfites andbisulfites in alkaline and acidic solutions, respectively, can beemployed. Notable examples are the alkali metals, e.g., sodium,potassium, etc.

Temperatures in the scrubber are not critical; however, if the inputgases are not quenched or otherwise reduced in temperature to about 50Cor below, the losses of ammonia due to volatilization begin to rise andthe scrubbing efficiency for rhenium extraction from the gases isreduced.

The effluent solution from the first scrubbing step is preferablyrecycled to the first scrubber to increase the concentration ofdissolved sulfites and rhenium oxides before the solution is bled offfrom the scrubber and treated for recovery of the rhenium oxides. Theincreased concentration of rhenium oxides in the solution permit greaterefficiency of operation in recovering the rhenium oxides from theeffluent solution.

The effluent solution from the first scrubbing step can be treated inany conventional manner to recover the dissolved rhenium oxidescontained therein. Forexample, the solution lends itself to treatmentwith ion exchange resins or conventional solvent extraction techniques.

The exit gas from the first scrubbing step, which may contain ammoniaand amounts of sulfur oxides which were not removed in the firstscrubber, is introduced into a second scrubber in which the residualraffmate solution from the rhenium oxide recovery treatment is employedas the scrubbing solution. The residual solution still contains thedissolved ammonium sulfite. In addition to the exit gas from the firstscrubber, a second gas stream containing sulfur oxides is alsointroduced into the second scrubber. A typical sulfur oxide-bearing gasstream in the exit gas from a sulfuric acid plant. Other gas streamscontaining sulfur dioxide and/or sulfur trioxide can be employed. In thesecond scrubbing step, the sulfur oxides from the combined exit gas andsecond gas stream are extracted from the gas streams by the scrubbingsolution which is maintained at an acid pH, preferably about pH 4.5 toabout pH 6.5. In an acid medium, the ammonium sulfiteis converted to thebisulfite and any residual amounts of ammonia contained in the exit gasfrom the first scrubber are removed therefrom for virtually completeefficiency in the utilization of ammonium ions.

The exit gas from the second scrubber can be routed through a wetelectrostatic precipitator to remove any additional amounts of mist orvapor from the gases; or the gas can be vented directly to theatmosphere if desired.

The bisulfite-containing effluent solution from the second scrubber canbe treated'by conventional means to recover the solid bisulfite productor can be treated for the removal of SO to produce solid ammoniumsulfate useful as a fertilizer.

Whereas this invention is illustrated and described 6 herein withrespect to certain preferred procedures thereof, it is to be understoodthat many variations are possible without departing from the inventiveconcepts particularly pointed out in the claims.

I claim:

1. A process for recovering rhenium and sulfur values from effluentgases containing such values in the form of oxides, comprisingintimately contacting a stream of such a gas with an aqueous alkalinesolution containing an appropriate cation in amount effective to combinewith sulfur values and to form a soluble sulfite solution;

continuing said contacting while maintaining said sulfite solutionalkaline, thereby extracting a maximum quantity of rhenium values fromthe gas stream;

treating the resulting rhenium-bearing sulfite solution for the recoveryof its contained rhenium values to produce a sulfite solutionsubstantially free of rhenium; and

intimately contacting a second, substantially rhenium-free stream ofgas, that contains a sulfur oxide, with the effluent sulfite solutionfrom the rheniumrecovery step, so as to remove sulfuroxide from saidsecond stream of gas and convert the sulfite in said solution to abisulfite.

2. A process as set forth in claim 1, wherein sulfur values arerecovered from the bisulfite solution resulting from the second gascontacting step.

3. A process as set forth in claim 2, wherein the aqueous alkalinesolution is ammonium hydroxide; and wherein the sulfur values recoveredfrom the bisulfite solution comprises ammonium bisulfite.

4. A process as set forth in claim 2, wherein the aqueous alkalinesolution is ammonium hydroxide; and wherein the sulfur values recoveredfrom the bisulfite solution comprises sulfur dioxide and ammoniumsulfate.

5. A process as set forth in claim 2, wherein the solutions are recycledback to and through the respective gas contacting steps beforerecovering the respective values therefrom.

6. A process as set forth inclaim 1, wherein the cation is selected fromthe group consisting of ammonium and alkali metals.

7. A process as set forth in claim 1, wherein the con tacting steps arecarried out by gas scrubbing procedures.

8. A process as set forth in claim 1, wherein the first gas stream isthe effluent gas from a reverberatory furnace in which copper sulfideore concentrates are being smelted.

9. A process as set forth in claim 8, wherein the .second gas stream isthe exit gas from a sulfuric acid plant.

10. A process as set forth in claim 1, wherein the pH of the solution inthe first contacting step lies within the range of about pH 8 and pH 10.

11. A process as set forth in claim 7, wherein the pH of the solution inthe second contacting step lies within the range of about pH 4.5 toaboutpH 6.5. g

12. A process as set forth in claim 1, wherein the cation is introducedinto the aqueous solution prior to contacting the first gas stream withsaid solution.

'13; Aprocess as set forth in claim 1, wherein the first contacting stepis carried out by gas scrubbing procedures, and the cation is ammoniumand is introduced into the solution by introducing ammonia gas into thescrubbing zone.

2. A process as set forth in claim 1, wherein sulfur values arerecovered from the bisulfite solution resulting from the second gascontacting step.
 3. A process as set forth in claim 2, wherein theaqueous alkaline solution is ammonium hydroxide; and wherein the sulfurvalues recovered from the bisulfite solution comprises ammoniumbisulfite.
 4. A process as set forth in claim 2, wherein the aqueousalkaline solution is ammonium hydroxide; and wherein the sulfur valuesrecovered from the bisulfite solution comprises sulfur dioxide andammonium sulfate.
 5. A process as set forth in claim 2, wherein thesolutions are recycled back to and through the respective gas contactingsteps before recovering the respective values therefrom.
 6. A process asset forth in claim 1, wherein the cation is selected from the groupconsisting of ammonium and alkali metals.
 7. A process as set forth inclaim 1, wherein the contacting steps are carried out by gas scrubbingprocedures.
 8. A process as set forth in claim 1, wherein the first gasstream is the effluent gas from a reverberatory furnace in which coppersulfide ore concentrates are being smelted.
 9. A process as set forth inclaim 8, wherein the second gas stream is the exit gas from a sulfuricacid plant.
 10. A process as set forth in claim 1, wherein the pH of thesolution in the first contacting step lies within the range of about pH8 and pH
 10. 11. A process as set forth in claim 7, wherein the pH ofthe solution in the second contacting step lies within the range ofabout pH 4.5 to about pH 6.5.
 12. A process as set forth in claim 1,wherein the cation is introduced into the aqueous solution prior tocontacting the first gas stream with said solution.
 13. A process as setforth in claim 1, wherein the first contacting step is carried out bygas scrubbing procedures, and the cation is ammonium and is introducedinto the solution by introducing ammonia gas into the scrubbing zone.